Segment Information - Reconciliation of Operating Revenues (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ 32,915	$ 31,543	$ 99,546	$ 93,600
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	31,017	29,485	92,835	87,526
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	2,009	2,203	7,093	6,442
Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ (111)	$ (145)	$ (382)	$ (368)